Above Ceiling Return Air Plenum

Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum.

Above ceiling return air plenum.

They also reduce the amount of energy used and are self balancing so they remove the need for balancing labor. Thermal insulation in return plenum areas would best be added at the roof and side walls. Shafts can be smaller as the complete open section of the shaft and ceiling are accessible for the return airflow. Sonobatts insulation can also.

Return air plenums also normally lessen the necessary depth of ceiling space. The air conditioning system supplies heated or cooled conditioned air through ducts in the above ceiling or attic space and this air comes into the room through a device called a register vent or diffuser. This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. Return air is then directed into the grille and travels to each crac unit.

Any large space designated for any type of air flow or routing of systems can be called a plenum. The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum. However some drop ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air handling space. The term plenum could be a large section of ductwork or the upper space above a ceiling.

This is most common in commercial buildings schools hospitals or libraries. Above the insulation should not act as a return air plenum.